Memory devices of two terminals in prior art have the following structure. A tunnel insulating film which is formed of a silicon oxide film is disposed on an interconnection that is formed of a conductor or an n-type (p-type) semiconductor. Silicon nanocrystals (silicon minute particles) which satisfy a Coulomb blockade condition (charging energy of an electron is larger than heat fluctuation) are formed on the tunnel insulating film such that the nanocrystals are spaced from each other.
A tunnel insulating film which is formed of a silicon oxide film is formed on a surface of the nanocrystals, and a silicon nitride film that has a silicon-rich composition is formed on the tunnel insulating film. In addition, an upper interconnection which is formed of a conductor or an n-type (p-type) semiconductor is formed on the silicon nitride film.
In the memory device structure described above, a current can flow between the surfaces of the interconnections by trap conduction in the silicon nitride film which includes a number of trap levels. Since the current increases and decreases according to presence/absence of trapped electrons in traps, the memory device can operate as a memory by reading the current between electrodes, using electrons which are trapped in traps located in the vicinity of a silicon nanocrystal that satisfies the Coulomb blockade condition, as information electric charges.
In the memory device described above, however, the current value between the interconnection electrodes, and leakage of the information electric charges trapped in traps in the vicinity of the silicon nanocrystal depend on trap conduction in the silicon nitride film (trap insulating film), and thus any memory characteristics depend on the trap state in the insulating film, which is difficult to control.